This invention relates to an optical recording method capable of high density recording on optical recording media such as phase change optical recording media, and phase change optical recording media to which the optical recording method is applicable.
Great attention is now paid to optical recording media capable of high density recording and erasing the once recorded information for rewriting. Among such rewritable optical recording media, phase change recording media are designed such that recording is performed by irradiating a laser beam to a recording layer to change its crystalline state and reading is performed by detecting the change of reflectivity of the recording layer associated with that state change. The phase change recording media are of greater interest because the drive unit used for their operation may have a simple optical system as compared with that used for magneto-optical recording media.
For the phase change recording layer, calcogenide materials such as Gexe2x80x94Sbxe2x80x94Te are often used because of a greater difference in reflectivity between the crystalline and amorphous states and a relatively high stability in the amorphous state.
When information is recorded in a phase change optical recording medium, the recording layer is irradiated with a laser beam having a high power (recording power) sufficient to heat the recording layer at or above its melting point. In the region where the recording power is applied, the recording layer is melted and then rapidly cooled, forming a recorded mark in the amorphous state. To erase the recorded mark, the recording layer is irradiated with a laser beam having a relatively low power (erasing power) sufficient to heat the recording layer above its crystallization temperature, but below its melting point. The recorded mark to which the erasing power is applied is heated above the crystallization temperature and then slowly cooled, resuming the crystalline state. Therefore, the phase change optical recording medium allows for overwriting simply by modulating the intensity of a single laser beam.
In order to increase the recording density and transfer rate of a recording medium, attempts have been made to reduce the wavelength of recording/reading beam, to increase the numerical aperture of an objective lens in a recording/reading optical system, and to increase the linear velocity of the medium. When a recording laser beam is irradiated to a medium rotating at a linear velocity V, the recording laser beam defines on the surface of the recording layer a spot having a diameter represented by xcex/NA wherein xcex is the wavelength of the laser beam and NA is the numerical aperture of the objective lens. The spot diameter xcex/NA divided by the linear velocity V, i.e., (xcex/NA)/V gives the time of irradiation of laser beam to the recording layer, that is, the time taken for passage across a beam spot. As the recording density and transfer rate increase, the irradiation time of laser beam to the recording layer becomes shorter and shorter. This makes it difficult to optimize overwriting conditions.
Problems arising from overwriting at an increased linear velocity are discussed below.
An increased linear velocity leads to a shortened irradiation time of recording beam. It is then a common practice to increase the recording power in proportion to the increased linear velocity for preventing the heated temperature of the recording layer from lowering. However, as the linear velocity increases, the rate of cooling following recording beam irradiation increases. To form an amorphous recorded mark, the recording layer, once melted by recording beam irradiation, must be cooled at or above a rate corresponding to the crystallization speed. For the given construction of recording layer and the given thermal design of medium, the cooling rate of the recording layer depends on the linear velocity. The cooling rate becomes higher at a higher linear velocity and lower at a lower linear velocity.
On the other hand, to erase the amorphous recorded mark (to recrystallize), erase beam must be irradiated such that the recording layer may be held for at least a predetermined time at a temperature between the crystallization temperature and the melting point. An attempt to increase the erasing power in proportion to the increased linear velocity for preventing the heated temperature of the recording layer from lowering has a less likelihood to erase the recorded mark because the irradiation time is reduced as a result of the increased linear velocity.
Therefore, to increase the linear velocity for improving the transfer rate, the recording layer must be formed of a composition having a relatively high crystallization speed such that recrystallization is completed within a relatively short time (as disclosed in JP-A 1-78444 and JP-A 10-326436), or the medium must be structured so as to retard heat release from the recording layer (slow cooling structure). It is also believed that the medium of slow cooling structure is also advantageous for preventing any drop of recording sensitivity which can otherwise occur as a result of an increased linear velocity, as described in JP-A 7-262613 and 8-63784.
The inventors made an experiment of overwriting on a phase change optical recording medium at a high transfer rate. The phase change optical recording medium used had a recording layer of a composition having a high crystallization speed so as to enable erasion at a high linear velocity and was structured for slow cooling. An attempt was made to gradually reduce the window margin Tw in order to increase the transfer rate. It became difficult to reduce the jitter when the signal length corresponding to the shortest recorded mark which was reduced in accordance with the reduction of window margin Tw was reduced below a certain value.
An object of the invention is to provide an optical recording method capable of increasing a transfer rate while minimizing the jitter. Another object of the invention is to provide an optical recording medium to which the method is applicable.
These and other objects are achieved by the invention which is defined below.
(1) An optical recording method for recording information in an optical recording medium having a recording layer by irradiating a laser beam thereto through an optical system, wherein provided that the laser beam has a wavelength xcex, the optical system includes an objective lens having a numerical aperture NA, the window margin is Tw, and a signal length corresponding to the shortest recorded mark is nxc2x7Tw, the recording is carried out under the conditions: xcex/NAxe2x89xa6680 nm, and nxc2x7Twxe2x89xa622 ns.
(2) The optical recording method of (1) wherein provided that the laser beam used to form the shortest recorded mark has an emission time Tmin, the recording is carried out under the condition: 0.113xe2x89xa6Tmin/(nxc2x7Tw)xe2x89xa61.0.
(3) The optical recording method of (1) or (2) wherein said recording layer is a phase change recording layer.
(4) The optical recording method of any one of (1) to (3) wherein the recording is carried out at a linear velocity V which satisfies the condition:(xcex/NA)/Vxe2x89xa660 ns.
(5) The optical recording method of any one of (1) to (4) wherein said optical recording medium includes the recording layer, a dielectric layer, and a reflective layer stacked in the order from closer to remoter one as viewed from the incident side of the recording laser beam,
the reflective layer has a thermal conductivity KR of at least 100 W/mK, and the dielectric layer disposed between the recording layer and the reflective layer has a thermal conductivity K2D of at least 1 W/mK.
(6) The optical recording method of any one of (1) to (5) wherein said optical recording medium includes a light-transmitting substrate, a dielectric layer, and the recording layer stacked in the order from closer to remoter one as viewed from the incident side of the recording laser beam, wherein
the dielectric layer disposed between the light-transmitting substrate and the recording layer includes at least two dielectric sublayers, of which two dielectric sublayers having different thermal conductivity are contiguous to each other,
one of the two dielectric sublayers which is disposed close to the recording layer has a thermal conductivity KC, and the other of the two dielectric sublayers which is disposed remote from the recording layer has a thermal conductivity KD wherein KC less than KD.
(7) The optical recording method of any one of (1) to (6) wherein the recording layer is represented by an atomic ratio composition:
(RaTebSbc)1-xMx
wherein R denotes rare earth elements, Te is tellurium, Sb is antimony, and M denotes constituent elements excluding R, Te and Sb, and letters a, b, c and x satisfy: a+b+c=1, a greater than 0, 0.4xe2x89xa6cxe2x89xa60.95, a/bxe2x89xa61.2, a/cxe2x89xa60.7, and 0xe2x89xa6xxe2x89xa60.1.
(8) An optical recording medium comprising a phase change recording layer represented by an atomic ratio composition:
(RaTebSbc)1-xMx
wherein R denotes rare earth elements, Te is tellurium, Sb is antimony, and M denotes constituent elements excluding R, Te and Sb, and letters a, b, c and x satisfy: a+b+c=1, a greater than 0, 0.4xe2x89xa6cxe2x89xa60.95, a/bxe2x89xa61.2, a/cxe2x89xa60.7, and 0xe2x89xa6xxe2x89xa60.1.
(9) The optical recording medium of (8) wherein said optical recording medium includes the recording layer, a dielectric layer, and a reflective layer stacked in the order from closer to remoter one as viewed from the incident side of a recording laser beam,
the reflective layer has a thermal conductivity KR of at least 100 W/mK, and the dielectric layer disposed between the recording layer and the reflective layer has a thermal conductivity K2D of at least 1 W/mK.
(10) The optical recording medium of (8) or (9) wherein said optical recording medium includes a light-transmitting substrate, a dielectric layer, and the recording layer stacked in the order from closer to remoter one as viewed from the incident side of the recording laser beam, wherein
the dielectric layer disposed between the substrate and the recording layer includes at least two dielectric sublayers, of which two dielectric sublayers having different thermal conductivity are contiguous to each other,
one of the two dielectric sublayers which is disposed close to the recording layer has a thermal conductivity KC, and the other of the two dielectric sublayers which is disposed remote from the recording layer has a thermal conductivity KD wherein KC less than KD.
(11) An optical recording medium comprising a phase change recording layer, a dielectric layer, and a reflective layer stacked in the described order as viewed from the incident side of a recording laser beam, wherein
the reflective layer has a thermal conductivity KR of at least 100 W/mK, and the dielectric layer disposed between the recording layer and the reflective layer has a thermal conductivity K2D of at least 1 W/mK.
(12) The optical recording medium of (11) further comprising another dielectric layer disposed in front of the recording layer as viewed from the incident side of a recording laser beam,
the other dielectric layer includes at least two dielectric sublayers, of which two dielectric sublayers having different thermal conductivity are contiguous to each other,
one of the two dielectric sublayers which is disposed close to the recording layer has a thermal conductivity KC, and the other of the two dielectric sublayers which is disposed remote from the recording layer has a thermal conductivity KD wherein KC less than KD.
(13) An optical recording medium comprising a light-transmitting substrate, a dielectric layer, and a phase change recording layer stacked in the order from closer to remoter one as viewed from the incident side of a recording laser beam, wherein
the dielectric layer disposed between the substrate and the recording layer includes at least two dielectric sublayers, of which two dielectric sublayers having different thermal conductivity are contiguous to each other,
one of the two dielectric sublayers which is disposed close to the recording layer has a thermal conductivity KC, and the other of the two dielectric sublayers which is disposed remote from the recording layer has a thermal conductivity KD wherein KC less than KD.
In the experiment of carrying out overwriting at a high transfer rate, the inventors have found that when the signal length corresponding to the shortest recorded mark, nxc2x7Tw (sometimes referred to as shortest signal length) is below a specific value, the shortest recorded mark is substantially deformed by self-erasing, resulting in an increased jitter. The self-erasing is described below.
In the prior art, whether or not characteristics of a phase change optical recording medium are good was judged by examining whether or not a satisfactory C/N is available at the linear velocity used and whether or not a satisfactory erasability is achievable upon erasing. However, too high an erasability rather adversely affects the characteristics. A high erasability means that the crystallization speed of the recording layer is fully high at the linear velocity used. In the recording layer having a high crystallization speed, a phenomenon occurs that when a trailing end portion of the recorded mark is being formed, for example, another portion, especially a leading end portion of the recorded mark is slowly cooled and recrystallized due to diffusion of heat in the lateral direction of the recording layer. Namely, the recorded mark is partially erased. This phenomenon is designated xe2x80x9cself-erasingxe2x80x9d in this specification. Since a medium having too high an erasability experiences a lowering of C/N and an increase of jitter owing to the self-erasing, a need exists to optimize the erasability. For example, JP-A 9-7176 describes a method of dividing a recording signal pulse and optimizing the pulse division pattern in accordance with the linear velocity, for preventing self-erasing from occurring when a medium designed for a high linear velocity is operated at a low linear velocity. The division pattern of recording signal pulse is generally referred to as recording pulse strategy.
In the above-mentioned experiment of carrying out overwriting at a high transfer rate, the inventors set the recording pulse strategy so as to minimize the jitter. However, as the shortest signal length nxc2x7Tw was shortened, it became impossible to divide the recording signal pulse during formation of the shortest recorded mark, owing to restrictions by the response or rise and fall characteristics of a laser beam emitting element. It was then impossible to restrain the jitter within an acceptable range when nxc2x7Tw was shortened.
Continuing further experiments, the inventors have found that in the event that the recording signal pulse used to form the shortest recorded mark cannot be divided, that is, a single pulse has to be used, the self-erasing may be alleviated by reducing the ratio of the recording signal pulse width to the shortest signal length nxc2x7Tw. However, the need to use a single pulse as the recording signal pulse arises in the situation that the shortest signal length nxc2x7Tw is short. To reduce the ratio of the recording signal pulse width to the shortest signal length nxc2x7Tw in that situation, the recording signal pulse width must be significantly reduced. On the other hand, the recording signal pulse width, that is, the light emission time of a laser beam emitting element (designated Tmin, hereinafter) cannot be extremely shortened owing to restrictions by the rise and fall characteristics of the laser beam emitting element. Therefore, there is a desire to have a medium which can reduce the jitter even when the light emission time Tmin is set relatively long, that is, when the ratio of the recording signal pulse width to the shortest signal length nxc2x7Tw is set relatively high.
Continuing further experiments based on the above experiments and considerations, the inventors have found that in the situation that the shortest signal length nxc2x7Tw is below a specific value and the recording signal pulse used to form the shortest signal is a single pulse, if a medium of rapid cooling structure is used, the jitter can be reduced even when the ratio Tmin/(nxc2x7Tw) of the light emission time Tmin to the shortest signal length nxc2x7Tw, is relatively high.
To enable erasing at a high linear velocity and to compensate for a drop of sensitivity due to the increased linear velocity, a slow cooling structure was customarily employed in the prior art as described in JP-A 7-262613 and 8-63784. However, if a medium of slow cooling structure is used in high transfer rate recording entailing a shortest signal length nxc2x7Tw of 22 ns or less, the jitter cannot be reduced as opposed to the present invention. It is noted that if a medium of rapid cooling structure as in the present invention is used and overwriting is carried out at a high linear velocity, a substantial drop of sensitivity occurs and the erasing of recorded marks becomes difficult.
In contrast, the present invention accommodates for the difficulty of erasing upon high linear velocity overwriting by controlling the composition of the recording layer. Also, by setting xcex/NAxe2x89xa6680 nm (wherein the recording beam has a wavelength xcex and the optical system objective lens has a numerical aperture NA), that is, by reducing the spot diameter of the laser beam used in overwriting, the energy density within the beam spot is increased to thereby compensate for the sensitivity drop. As a result, the invention is successful in reducing the jitter without incurring a drop of recording sensitivity and a drop of erasability.
It is noted that in the examples of the above-referenced JP-A 7-262613 and 8-63784, the objective lens has a numerical aperture of 0.5 and the laser wavelength is 780 nm. If the medium has a rapid cooling structure in such greater xcex/NA cases, the recording sensitivity lowers to interfere with recording, as demonstrated in these patent references.
The invention provides the medium with a rapid cooling structure by controlling the thermal conductivity of the reflective layer and the dielectric layer or by constructing the dielectric layer as a laminate structure including a plurality of dielectric sublayers having different thermal conductivity.
The invention remains effective independent of a linear velocity as long as the shortest signal length nxc2x7Tw is not greater than the specific value. It is noted that as the linear velocity becomes higher, the crystallization speed of the recording layer must be increased high enough to erase recorded marks and as a result, the influence of self-erasing becomes substantial. For this reason, the invention is especially effective upon high linear velocity recording.
As described above, the invention is effective when the irradiation time of laser beam to the recording layer is short so that the crystallization speed of the recording layer must be increased. Specifically, the invention is effective when the beam spot passage time (xcex/NA)/V serving as an index of laser irradiation time is not greater than 60 ns, that is, (xcex/NA)/Vxe2x89xa660 ns.
Although the invention is effective particularly for phase change recording media, it is also applicable to other optical recording media of heat mode recording, such as magneto-optical recording media. In magneto-optical recording media as well, the recording pulse strategy is utilized in order to control heat transfer in the lateral direction of the recording layer. However, if the transfer rate becomes increased, the pulse division becomes impossible for the shortest recorded mark and so, the jitter becomes increased. By applying the invention to such media, the jitter can be reduced.